The pathophysiological mechanisms that underlie metabolic bone diseases due to impaired phosphate homeostasis are incompletely understood. Investigating the molecular etiology of disorders caused by disturbed mineral metabolism has been instrumental in identifying new circulating regulators of phosphate homeostasis, collectively referred to as `phosphatonins.'We identified the phosphatonin Fibroblast growth factor-23 (FGF23) in a positional cloning approach to isolate the gene for autosomal dominant hypophosphatemic rickets (ADHR), characterized by hypophosphatemia secondary to renal phosphate wasting, rickets/osteomalacia and fracture. Building upon this work during the current grant cycle, our group discovered the molecular basis for a novel recessive phosphate wasting disorder associated with elevated FGF23, as well as determined that the mirror-image disorder to ADHR, familial tumoral calcinosis (TC), is caused by recessive FGF23 loss of function mutations. A TC phenotype is also present in the Klotho (KL)-null mouse, and our data demonstrate that FGF23 elicits its biological actions through this receptor-like molecule. However, the KL isoform that mediates FGF23-dependent bioactivity is unknown. Our preliminary findings using a novel knock in mouse carrying a human ADHR Fgf23 mutation demonstrate the presence of unique skeletal alterations, providing new insight into FGF23 function in vivo. Although significant progress has been made determining the roles of FGF23 in normal circumstances and in disease, the mechanisms whereby FGF23 controls phosphate handling remain unclear. Thus, the central hypothesis to be tested within this proposal is: FGF23 is secreted from bone cells in a regulated manner and acts through specific target molecules to control phosphate homeostasis in the kidney. We will address this hypothesis by undertaking the following specific aims: To test the molecular mechanisms underlying ADHR using a murine knock in model;to determine the role of KL in the renal proximal tubule and distal tubule actions of FGF23;and to test the KL isoforms for the ability to direct FGF23 bioactivity in vivo. These investigations will, over the long term, provide critical insight into the pathogenesis of syndromes associated with altered FGF23 expression, into more common disorders of disturbed phosphate homeostasis, as well as into the basic biology of phosphate homeostasis.